Machines of the type defined hereinabove are already known.
Thus, FIG. 1 represents a machine 1 of the prior art in a production line 2 for single-face corrugated board.
The line comprises feed means 3 and 4 respectively for plane sheet, cover sheet and plane sheet intended to form the fluted sheet.
These feed means comprise, in a manner which is known per se, unwinders for reels 5 which allow good control of the unwinding and of the braking which are necessary for the manufacture of the board.
They also comprise a preheater 6 for the cover sheet, generally consisting of a steel cylinder heated by steam and fitted with small rolls called "turn rolls" which serve to alter the paper/cylinder contact surface area, and a preconditioner 7 for the fluted sheet, which, for its part, furthermore comprises a boom for wetting the sheet, which promotes the formation of the flutes.
The line 2 moreover comprises means 8 for removing the single-face board obtained, which means consist of a system of belts at the top part of the line.
FIG. 2 more precisely shows, in section, the single-face machine 1 of the line in FIG. 1.
It comprises, on the fluted-sheet feed side, an additional preheating cylinder 9 and a wetting cylinder 10, and, on the cover-sheet feed side, two rotating preheating cylinders 11.
The machine 1 moreover comprises a first, upper, fluted cylinder 12 made of stainless steel. It is hollow and arranged to be heated by steam in a manner which is known per se.
The machine 1 also comprises a second, fluted, central cylinder 13 made of stainless steel, of axis parallel to that of the first cylinder substantially tangent to the latter and, for example, of the same diameter.
The second cylinder is, for example, of the type known by the brand name "Air Drive" manufactured by the French company MARTIN.
It comprises two chambers, namely a central chamber heated by steam and a vacuum chamber 15 connected to a depressurization device 16. The chamber comprises channels 17 pierced over the entire length of the cylinder and communicating with the fluted peripheral surface via holes.
The machine 1 comprises a third, lower cylinder 18 of smooth surface, of axis parallel to the first two and, for example, of the same diameter. This cylinder is heated by steam in a manner similar to the first two and is arranged to compress the cover sheet against the ridges of the flutes of the fluted sheet, in contact with the periphery of the central fluted cylinder, as will be seen further on.
The machine 1 moreover comprises means 19 for gluing the crests of the flutes, which means are known per se, comprising a calender roll 20, an adhesive tank 21 and a gluing roll 22.
The principle of operation of the machine 1 is as follows.
Two sheets or layers of paperboard 23 and 24 are introduced into the machine 1.
The sheet 23 is intended to form the fluted sheet.
After drying and wetting treatment at 7, 9 and 10, it passes around the first fluted heating cylinder 12 over a part of its periphery.
It is then introduced between the two fluted heating cylinders 12 and 13 which turn in mutually opposite directions as two meshing cylinders.
The vacuum created in the lower cylinder 13 then presses the fluted sheet 23 formed against the heating cylinder over a circle arc having a vertex angle equal to of the order of 180.degree..
During passage in front of the adhesive-application roll 22, a line of adhesive is, moreover and as has been seen, deposited on the crest of the flutes.
The adhesive is, for example, based on starch.
The cover sheet 24 is, for its part, introduced at the bottom and on the opposite side of the machine 1.
It is predried at 11 and wound around the third cylinder 18, also called the smooth press. A very high hydraulic pressure, for example 5 kg/cm (linear pressure) ensures successive contact at 25 (see FIG. 3) between each upper ridge of the flutes of the fluted sheet 23 and the cover sheet 24 made of smooth paper.
The adhesive joint is therefore obtained by the combined action of the high pressure and the temperature in a fraction of a second.
The fluted sheet is moved by the meshing of the fluted cylinders 12 and 13, and the cover sheet is moved by clamping between the central cylinder 13 and the smooth cylinder 18, no tension being exerted downstream of the three cylinders. The corrugated board is subsequently removed at the top, in a manner which is known per se, by a conveyor belt.
Other types of single-face machines exist, such as, for example, those in which the fluted sheet is pressed flat by air instead of being sucked by vacuum onto the central cylinder.
Their principle of operation and the structural elements which they employ remain, however, substantially identical to those described hereinabove.
Although they make it possible to achieve high throughput while giving acceptable single-face paper, known machines nevertheless still have drawbacks.
In particular, in order to obtain good adhesive bonding, which is the aim sought if high-quality corrugated board is desired, it has to date essentially been considered necessary to apply the two sheets against one another with a very high pressure, moreover given that a sufficient temperature is maintained at the moment of the adhesive bonding in order to allow gelatinization of the adhesive.
Unfortunately, and importantly, this pressure has detrimental effects.
It is in fact applied onto the upper part of the crests of the flutes (see FIG. 3), which causes cuts in the board, in particular at resonant speeds, or even at high speeds.
Furthermore, since the application of the press cylinder 18 onto the central fluted cylinder 13 takes place discontinuously, from one flute to the next, vibrations and intense noise (105 to 110 decibels at high speed) are generated.
This results in poor-quality corrugated board, in which the characteristics of resistance to moisture and bursting of the cover, for example, are degraded.
It is also known (EP 0559556) a device for manufacturing corrugated board comprising means allowing the wrap around of the board according to a winding angle on the central fluted cylinder and means for heating such board situated downstream.
Such a device is only directed to be used with boards connected to each other end to end and excludes any possibilities of tension or traction on the liner board per se.
Also, it is known (U.S. Pat. No. 2,638,962 and U.S. Pat. No. 4,480,066) to use belts for driving away the paper when the corrugated board is formed, or a vacuum table to avoid any shearing strength between the liner board and the fluted board at the level of the glue joints.
Such device and process need however to press the paper on a plurality of adjacent flutes of the central fluted cylinder, which involves systematic wear and the usual difficulties to use such systems.